1. Field of the Invention
The present invention relates generally to the field of actuators and, more particularly, to an electro-pneumatic actuator having high frequency response and capable of supporting high forces, and a servo-valve for use therewith.
2. Description of Related Art
Actuators are used in mechanical systems to isolate sensitive components from external vibrational forces. Specifically, actuators used in combination with control circuitry employing feed-back or feed-forward loops can dampen the external vibrational forces, thus isolating a load from the source of the vibration. For example, a complex optical structure such as a large telescope or a large laser mounted in an aircraft would be subject to vibrations caused by the aircraft engines and by air turbulence. Such vibrations can adversely affect the operation and longevity of-the optical components. Thus, it is necessary to isolate the optical structure from the vibrations, and such isolation can be accomplished by the use of actuators. Also, actuators having push-pull capabilities (i.e., having operative control in opposite directions) can also be used for steering or direction control, as well as isolation.
The vibrations caused by the aircraft engines are relatively high frequency vibrations on the order of 200-300 Hertz (Hz), whereas the vibrations caused by air turbulence are on the order of 2-5 Hz. When high frequency response is desired, electromagnetic actuators are employed but are generally not suitable for heavy loads, such as large telescopes or laser systems which may weigh up to 10,000 pounds. For use with such heavy loads, hydraulic or pneumatic actuators are preferred, but they perform poorly at high frequencies. For the present example of an optical structure in an aircraft environment, both high frequency response and high forces are required.
Currently, known actuators which can deliver high forces at high frequencies are either hydraulic actuators or very large electromagnetic actuators. Hydraulic actuators suffer from high stiffness at high frequencies, however, which reduces their effectiveness at isolating their payloads. Also, hydraulic actuators tend to leak, making them unsuitable for clean systems applications, such as optical systems in which leaking hydraulic fluid can adversely affect the performance of the optical components.
Large electromagnetic actuators, on the other hand, have high power consumption, especially when supporting heavy loads. In addition, the frequency response suffers if the electromagnetic actuators are designed to support such a large load. Electromagnetic actuators are therefore impractical for the present application.
One solution is to use an electromagnetic actuator in series with a "stiff actuator" such as a ball screw or a hydraulic cylinder. The electromagnetic actuator typically used is a variable reluctance actuator in order to obtain the required forces. However, this type of electromagnetic actuator has low frequency response and high internal inductance. The high inductance limits both the frequency response of the actuator, as well as the forces available at high frequencies due to the large back electromagnetic field (emf) generated by the inductance.
Thus, there is a need for an actuator which combines high frequency response and high force, and which does not become stiff at high frequencies. It is also desirable to have an actuator which has high frequency response and high force, which is not hydraulic.